The present invention relates generally to a polymer processing system and apparatus and, more particularly, to a counter-rotating twin screw extrusion system for the production of polymeric foam.
Polymeric foams include a plurality of voids, also called cells, in a polymer matrix. By replacing solid material with voids, polymeric foams use less raw material than solid plastics for a given volume. Thus, by using polymeric foams in certain applications instead of solid plastics, material costs may be reduced. It can be useful to characterize a foam by features of its cellular structure such as cell size, cell density, and the degree of cell interconnectivity. Microcellular foams (or microcellular materials) are a class of polymeric foams that have small cell sizes and high cell densities.
Polymeric foams can be produced using a number of known techniques. In an extrusion process, for example, foamed polymeric materials can be produced by introducing a physical blowing agent into a molten polymeric stream, mixing the blowing agent with the polymer, and extruding the mixture into the atmosphere while shaping the mixture. Exposure to atmospheric conditions causes the blowing agent to gasify, thereby forming cells in the polymer. Alternatively in the extrusion process, a chemical blowing agent can be added and caused to react in the molten polymeric stream, resulting in the generation of gas that forms cells in the polymer.
Counter-rotating twin screw extruders include two intermeshing screws that rotate in opposite directions, and are particularly useful for processing powders or polymeric materials that are susceptible to degradation, such as polyvinyl chloride (PVC). The counter-rotating screws have tight tolerances therebetween which promotes efficient downstream pumping of polymeric material with low shear forces and short residence times. Furthermore, typical counter-rotating screws reduce the tendency of polymeric material to recirculate, commonly referred to as leakage flow. degradation, they discourage the injection and uniform mixing of a physical blowing agent into the extruder, for example, as in certain foam processes. In particular, the low leakage flow limits the degree of mixing of the polymeric material in the extruder which may prevent proper dispersion of a physical blowing agent. Also, the low leakage flow makes it difficult to maintain pressure in the middle of the extruder, the typical point of blowing agent injection, which restricts the ability to quickly and uniformly dissolve the blowing agent in the polymeric material as needed in certain foam processes. Therefore, counter-rotating twin screw extrusion systems, to the applicants"" knowledge, have not been used for foam processing with physical blowing agent injection.
Accordingly, a need exists for extrusion systems that can be used to process polymeric foams and, in particular, foams of materials that are shear sensitive and susceptible to degradation during processing, and that also can rapidly and thoroughly mix physical blowing agent with polymeric material in the extruder.
The present invention provides a counter-rotating twin screw extrusion system for processing foam material. The system is designed to introduce a physical blowing agent into the extruder, thoroughly disperse the blowing agent within the polymer melt and, preferably, to maintain relatively high pressures downstream from the point of blowing agent introduction. High quality polymeric foams, including microcellular foams, can be produced using the extrusion system.
In one embodiment, the invention provides a polymer processing apparatus. The apparatus includes an extruder having a barrel constructed to house a pair of twin polymer processing screws mounted therein. The extruder is constructed and arranged to drive the screws counter-rotationally and to convey polymeric material within a polymer processing space in a downstream direction. The apparatus further includes a blowing agent injection port fluidly communicating with the barrel and connectable to a source of blowing agent for introducing a blowing agent into the polymeric material in the polymer processing space.
In another embodiment, the invention provides a system. The system includes a pair of twin polymer processing screws constructed and arranged for counter-rotation within a barrel of a polymer processing apparatus. Each of the screws includes a blowing agent injection section positionable adjacent a blowing agent injection port in the barrel when the screws are mounted within the barrel.
In another embodiment, the invention provides a system. The system includes a pair of twin polymer processing screws constructed and arranged for counter-rotation within a barrel of a polymer processing apparatus. Each of the screws includes a mixing section positionable downstream of a blowing agent injection port in the barrel when the screws are mounted within the barrel.
In another embodiment, the invention provides a method. The method includes conveying polymeric material admixed with a blowing agent in a downstream direction in an extruder with counter-rotating screws.
Among other advantages, the invention provides a counter-rotating twin screw extruder into which a physical blowing agent can be introduced. The extruder, therefore, can be used to produce polymeric foams. The system utilizes the advantages of the counter-rotating screw configuration, such as efficient pumping with low shear and short residence times, thus making it particularly suitable for the production of foams using shear sensitive and degradable materials.
Furthermore, in certain embodiments, the extrusion system is configured to produce microcellular material. Microcellular materials have smaller cell sizes and higher cell densities than conventional polymeric foams. The unique cell structure of microcellular foams leads to several advantages over conventional foams including improved properties and appearance, amongst others.
As used herein, microcellular material, or microcellular foam, is defined as material having an average cell size of less than 100 microns or a cell density of less than 106 cells/cm3, and preferably both.
As used herein, cell density is the number of cells per cubic centimeter of unexpanded, solid plastic.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although systems and materials similar to those described herein can be used in the practice for testing of the present invention, suitable systems and materials are described below. All publications, patent applications, patents, and other references incorporated by reference herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the systems, materials, and examples are illustrative only, and not intended to be limiting.
Other advantages, novel features, and aspects will become apparent from the following detailed description when considered in conjunction with the accompanying figures and from the claims. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.